Good relations between Poland and Hungary date back to the Middle Ages. The Polish and Hungarian noble houses (as Piast dynasty or House of Árpád) often intermarried with each other. Louis the Great was King of Hungary and Croatia from 1342 and King of Poland from 1370 until his death in 1382. He was his father’s heir, Charles I of the House of Anjou-Sicily (King of Hungary and Croatia) and his uncle’s heir, Casimir III the Great (King of Poland - the last ruler of Piast dynasty). King Casimir had no legitimate sons. Apparently, in order to provide a clear line of succession and avoid dynastic uncertainty, he arranged for his nephew, King Louis I of Hungary, to be his successor in Poland. Louis' younger girl Saint Jadwiga of Poland inherited the Polish throne, and became one of the most popular monarch of Poland. In the 15th century, the two countries briefly shared the same king again, Poland's Władysław III of Varna, who perished, aged barely twenty, fighting the Turks at Varna, Bulgaria. In the 16th century, Poland elected as her king a Hungarian nobleman, Stefan Batory, who is regarded as one of Poland's greatest kings.

Hungarian Revolution of 1848

In the Hungarian Revolution of 1848, a Polish general, Józef Bem, became a national hero of both Hungary and Poland. He was entrusted with the defence of Transylvania at the end of 1848, and in 1849, as General of the Székely troops. On October 20, 1848 Józef Wysocki signed an agreement with the Hungarian government to form a Polish infantry battalion of about 1,200 soldiers. After agreement Wysocki organized in Hungary "Polish legion" of volunteers contained 2,090 foot soldiers and 400 Polish uhlans. They took part in the siege of the Arad fortress in the spring of 1849 and participated in all important battles at Szolnok, Hatvan, Tápió-Bicske and Isaszeg.

Polish Soviet war

During the Polish–Soviet War (1919–21), after the Bela Kun government in Hungary was overthrown, Hungary offered to send 30,000 cavalry to Poland's aid, but the Czechoslovak government refused to allow them through the demilitarized zone that had existed between Czechoslovakia and Hungary since the end of the Czechoslovak-Hungarian war a few months earlier. Nevertheless, some Hungarian munitions trains did reach Poland.

Revolution of 1956

A student demonstration in Budapest in support of the Polish October and asking for similar reforms in Hungary was one of the events that sparked the Hungarian Revolution of 1956. During the revolution, Poles demonstrated their support for the Hungarians by donating blood for them; by 12 November 1956, 11,196 Poles had donated. The Polish Red Cross sent 44 tons of medical supplies to Hungary by air; even larger amounts were sent by road and rail.

Friendship Day

On March 12, 2007, Hungary's parliament declared March 23 the "Day of Hungarian-Polish Friendship", with 324 votes in favor, none opposed, and no abstentions. Four days later, the Polish parliament declared March 23 the "Day of Polish-Hungarian Friendship" by acclamation. source

I know. It's mostly because the fandom doesn't pay much attention to Liechtenstein's history (especially for a small country that's never noticed). It's heavily influenced by Austrian culture and their relations are still good today.